The present invention relates to communications devices, infrastructure equipment, mobile communications systems and methods for receiving downlink control information at a communications device transmitted from a mobile communications network.
Certain classes of telecommunications device, such as MTC devices (e.g. semi-autonomous or autonomous wireless communication terminals), support “low capability” communication applications that are characterised, for instance, by the transmission of small amounts of data at relatively infrequent intervals. It is preferable that MTC devices are constructed so that individually they represent little burden on telecommunications networks and thus can be deployed in greater numbers than equivalent “full capability” terminals in the same networks.
In many scenarios, it is preferable to provide terminals dedicated to such “low capability” communication applications with a simple receiver unit (or transceiver unit) having capabilities more commensurate with the amount of data likely to be transmitted to (or from) the terminal. This more limited capability contrasts with the capabilities of the conventional mobile telecommunications terminals, such as smartphones, which share access to the same telecommunications networks.
Fourth generation mobile communications systems are being developed in accordance with Long Term Evolution (LTE) standards by the Third Generation Project Partnership (3GPP). In conventional LTE, at least some of the resource elements (REs) comprising a control region are defined by specification to form a number of so-called control channel elements (CCEs) or enhanced control channel elements (ECCEs). A physical downlink control channel (PDCCH), for providing control information to devices, comprises a number of CCEs. An enhanced physical downlink control channel (EPDCCH), for providing control information to devices, comprises a number of ECCEs. The number of CCEs or comprising a particular PDCCH or EPDCCH depends on an aggregation level determined by an eNB of an LTE mobile communications network (see later for discussion of aggregation levels). A UE must search through some number of the (E)CCEs in the control region to determine if there are any that comprise PDCCHs containing control information pertinent to the UE. Some CCEs are searched by all UEs, these CCEs comprising a so-called common search space (CSS), and some (E)CCEs are not searched by all UEs, these (E)CCEs comprising so-called UE-specific search spaces (UESS). A (E)CCE may be part of more than one search space. Typically, (E)PDCCHs comprising (E)CCEs in the common search space contain information relevant to all UEs in a cell and (E)PDCCHs comprising (E)CCEs in a UE-specific search space contain information relevant only to one UE.
As will be appreciated it is desireable for communications devices and particularly but not exclusively reduced capability or MTC type devices to be arranged to consume as little power as possible in order to conserve power and therefore battery life and for the implementation of the device to possible with low complexity so that the modem can have low cost.